Ozone may affect leaf photosynthesis even before visible symptoms become apparent. This study had the objective to test several parameters of chlorophyll fluorescence and leaf gas exchange for their usefulness as indicators of latent ozone injury in the field. Container-grown apple trees (Malus domestica Borkh. cv. Golden Delicious) were exposed to four different ozone levels in open-top chambers. Identical leaves were analyzed in fixed-time intervals for the characteristics of fast fluorescence induction kinetics in vivo. By using high-time resolution, characteristic parameters describing the early photochemical events could be calculated according to the JIP-test. Parameters responsive to the different ozone treatments showed clear dependence on the accumulated ozone dose. Ozone exposure immediately preceding the measurements was more important for the extent of the physiological effects than the total accumulated ozone dose. The most sensitive parameters were the turnover number N (indicating how many times QA has to be reduced for full reduction of all acceptors; positively correlated to ozone dose) and D0, the density of reaction centres per leaf area (negatively correlated to ozone dose). Most parameters analyzed showed clearer responses to ozone on the adaxial than on the abaxial surface of the leaf. Changes in the parameter N were better correlated to ozone doses with low cut-offs (AOT00 and AOT20), whereas changes in D0 and in the specific electron fluxes per reaction centre were mainly influenced by ozone doses with high cut-offs (AOT80 and AOT100). Leaf gas exchange analyses revealed a higher ozone sensitivity in carboxylation efficiency than in light utilization efficiency and in the rate of light-saturated net photosynthesis. All ozone-induced photosynthetic effects were observed in leaves showing no sign of visible leaf injury. This study identified fluorescence parameters that could be useful for rapid monitoring and early detection of latent leaf injury by ozone.